Various types of molded products have been manufactured by an injection molding machine. In the injection molding machine, generally, a molten resin is injected into a cavity formed by a fixed side die and movable side die and is cooled and solidified in the dies to be formed into a molded product. If there is a variation in the molding conditions, or differences occur in the temperature distribution and cooling speed depending on the location in the cavity, variations or distortion may occur to the molded products, and molding failures may occur.
To solve this problem, Patent Document 1, for example, discloses various measures to minimize the irregularity in temperature distribution for molding a long-shaped optical element. For instance, in Example 13 of the Patent Document 1, a molding die wherein a plurality of heaters and a controller for controlling the heaters are provided in the vicinity of the die cavity. It is said that this structure achieves a desired temperature distribution to prevent optical distortion.
However, the aforementioned conventional method is restricted to a long-shaped optical element. Another method of injection molding uses a multiple cavity die wherein a plurality of cavities are formed within one die. In the precision optical parts such an optical lens molded by this method, variations may occur among different molded products for each cavity if there is irregularity in the temperature distribution or cooling speed in the die.
Further, in the continuous molding operation mode, variations in molded products may occur due to a change in the outside temperature for each molding shot. A change in the outside temperature affects the die temperature to produce a subtle change in the molding conditions. Thus, variations in performances of the molded products may occur for each shot, in the conventional art.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 11-42682